Time-resolved absorption spectroscopy is used to study the dynamics of protein structural changes subsequent to excitation with short laser pulses. Molecular models for the protein dynamics are used to fit and interpret the measured data. A. The kinetics of ligand binding and conformational changes for sperm whale myoglobin and human hemoglobin (HbA) have been studied following the set of data from partial photolysis experiments using polarized excitation has permitted cooperative and noncooperative processes to be clearly distinguished. We are now proceeding to test specific molecular models for the rebinding dynamics. B. The photocycles of bacteriorhodopsin (bR), a photoactive proton pump have been investigated. A detailed analysis of data collected by Xie et al. has shown that back reactions are significant for all steps of the cycle except the last. The temperature- and pH-dependence of these rates is obtained from fits of the model to the data. C. The effect of polymerization kinetics on the morphology of gels of hemoglobin S. has been investigated by polarizing microscopy and computer simulations.